Re: [Qemu-devel] [PATCH RFC v4 19/20] intel_iommu: unmap existing pages before replay

[Peter Xu]

On Sun, Jan 22, 2017 at 04:13:32PM +0800, Jason Wang wrote:
> 
> 
> On 2017年01月20日 21:08, Peter Xu wrote:
> >Previous replay works for domain switch only if the original domain does
> >not have mapped pages. For example, if we switch domain from A to B, it
> >will only work if A has no existing mapping. If there is, then there's
> >problem - current replay didn't make sure the old mappings are cleared
> >before replaying the new one.
> 
> I'm not quite sure this is needed. I thought the only thing we need to do is
> stop DMA of device during the moving? Or is there an example that will cause
> trouble?

I think this patch is essential.

Example:

- device D1 moved to domain A, domain A has no mapping
- map page P1 in domain A, so D1 will have a mapping of page P1
- create domain B with mapping P2
- move D1 from domain A to domain B

Here if we don't unmap existing pages in domain A (P1), after the
switch, we'll have D1 with both P1/P2 mapped, while domain B actually
only has P2. That's unaligned mapping, and it should be wrong.

If you (or anyone) think this is a bug as well for patch 18, I can
just squash both 19/20 into patch 18.

Thanks,

-- peterx